Refrigeration systems have been in existence since the early 1900s, when reliable sealed refrigeration systems were developed. Since that time, improvements in refrigeration technology have proven their utility in both residential and industrial settings. In particular, low-temperature refrigeration systems currently provide essential industrial functions in biomedical applications, cryoelectronics, coating operations, and semiconductor manufacturing applications.
There are many important applications, especially industrial manufacturing and test applications, which require refrigeration at temperatures below 183 K (−90° C.). This invention relates to refrigeration systems that provide refrigeration at temperatures between 183 K and 65 K (−90° C. and −208° C.). The temperatures encompassed in this range are variously referred to as low, ultra low and cryogenic. For purposes of this application the term “very low” or “very low temperature” will be used to mean the temperature range of 183 K and 65 K (−90° C. and −208° C.).
In many manufacturing processes conducted under vacuum conditions, and integrated with a very low temperature refrigeration system, rapid heating is required in certain processing steps. This heating process is commonly referred to as a defrost cycle. The heating process warms the evaporator and connecting refrigerant lines to room temperature. This enables these parts of the system to be accessed and vented to atmosphere without causing condensation of moisture from the air on these parts. The longer the overall defrost cycle and subsequent resumption of producing very low temperature temperatures, the lower the throughput of the manufacturing system. Enabling a quick defrost and a quick resumption of the cooling of the cryosurface (evaporator) in the vacuum chamber is beneficial to increase the throughput of the vacuum process.
In addition, there are many processes where it is desired to provide a flow of hot refrigerant through the evaporator for an extended period of time. For purposes of this application, we refer to this as a “bakeout” operation. This is beneficial when the element being alternately heated and cooled by the refrigerant has a large thermal mass, and where the temperature response as a function of time is longer than about one to five minutes. In such cases, a prolonged flow of high temperature refrigerant is required to allow thermal conduction of the heat to occur until all surfaces reach the desired minimum temperature. In addition, a common procedure in vacuum chambers is a mode where the surfaces in the chamber are heated to high temperatures, typically of 150° C. to 300° C. Such high temperatures will radiate to all surfaces in the chamber, including the element cooled and heated by the refrigerant. Exposing the refrigerant and any residual compressor oil resident in the element to such high temperatures when no refrigerant flow is occurring through the element presents the risk of overheating the resident refrigerant with consequent decomposition of the refrigerant and/or the oil. Therefore, providing continuous flow of high temperature refrigerant (typically 80 to 120° C.), while the chamber is being heated, controls the temperature of the refrigerant and oil and prevents any possible decomposition.
There are many vacuum processes that have the need for such very low temperature cooling. The chief use is to provide water vapor cryopumping for vacuum systems. The very low temperature surface captures and holds water vapor molecules at a much higher rate than they are released. The net effect is to quickly and significantly lower the chamber's water vapor partial pressure. This process of water vapor cryopumping is very useful for many physical vapor deposition processes in the vacuum coating industry for electronic storage media, optical reflectors, metallized parts, semiconductor devices, etc. This process is also used for remove moisture from food products and biological products in freeze drying operations.
Another application involves thermal radiation shielding. In this application large panels are cooled to very low temperatures. These cooled panels intercept radiant heat from vacuum chamber surfaces and heaters. This can reduce the heat load on surfaces being cooled to temperatures lower than that of the panels. Yet another application is the removal of heat from objects being manufactured. In some applications the object is an aluminum disc for a computer hard drive, a silicon wafer for the manufacture of a semiconductor device, or a material such as glass or plastic for a flat panel display. In these cases, the very low temperature provides a means for removing heat from these objects more rapidly, even though the object's final temperature at the end of the process step may be higher than room temperature.
Further, some applications involving hard disc drive media, silicon wafers, or flat panel display material, or other substrates, involve the deposition of material onto these objects. In such cases heat is released from the object as a result of the deposition and this heat must be removed while maintaining the object within prescribed temperatures. Cooling a surface like a platen is the typical means of removing heat from such objects. In all these cases an interface between the refrigeration system and the object to be cooled is proceeding in the evaporator where the refrigerant is removing heat from the object at very low temperatures.
Still other applications of very low temperatures include the storage of biological fluids and tissues and control of reaction rates in chemical and pharmaceutical processes.
Conventional refrigeration systems have historically utilized chlorinated refrigerants, which have been determined to be detrimental to the environment and are known to contribute to ozone depletion. Thus, increasingly restrictive environmental regulations have driven the refrigeration industry away from chlorinated fluorocarbons (CFCs) to hydrochlorofluorocarbons (HCFCs). Provisions of the Montreal Protocol require a phase out of HCFC's and a European Union law bans the use of HCFCs in refrigeration systems as of Jan. 1, 2001. Therefore the development of an alternate refrigerant mixture is required. Hydroflurocarbon (HFC) refrigerants are good candidates that are nonflammable, have low toxicity and are commercially available.
Prior art very low temperature systems used flammable components to manage oil. The oils used in very low temperature systems using chlorinated refrigerants had good miscibility with the warmer boiling components that are capable of being liquefied at room temperature when pressurized. Colder boiling HFC refrigerants such as R-23 are not miscible with these oils and do not readily liquefy until they encounter colder parts of the refrigeration process. This immiscibility causes the compressor oil to separate and freezeout, which in turn leads to system failure due to blocked tubes, strainers, valves or throttle devices. To provide miscibility at these lower temperatures, ethane is conventionally added to the refrigerant mixture. Unfortunately, ethane is flammable, which can limit customer acceptance and can invoke additional requirements for system controls, installation requirements and cost. Therefore, elimination of ethane or other flammable component is preferred.
Refrigeration systems such as those described above require a mixture of refrigerants that will not freezeout from the refrigerant mixture. A “freezeout” condition in a refrigeration system occurs when one or more refrigerant components, or the compressor oil, becomes solid or extremely viscous to the point where it does not flow. During normal operation of a refrigeration system, the suction pressure decreases as the temperature decreases. If a freezeout condition occurs the suction pressure tends to drop even further creating positive feedback and further reducing the temperature, causing even more freezeout.
What is needed is a way to prevent freezeout in a mixed refrigerant refrigeration system. HFC refrigerants available have warmer freezing points than the HCFC and CFC refrigerants that they replace. The limits of these refrigerant mixtures with regard to freezeout are disclosed in U.S. application for patent Ser. No. 09/886,936. As mentioned above, the use of hydrocarbons is undesirable due to their flammability. However, elimination of flammable components causes additional difficulties in the management of freezeout since the HFC refrigerants that can be used instead of flammable hydrocarbon refrigerants typically have warmer freezing points.
Typically freezeout occurs when the external thermal load on the refrigeration system becomes very low. Some very low temperature systems use a subcooler that takes a portion of the lowest temperature high-pressure refrigerant and uses this to cool the high-pressure refrigerant. This is accomplished by expanding this refrigerant portion and using it to feed the low-pressure side of the subcooler. Thus when flow to the evaporator is stopped, internal flow and heat transfer continues allowing the high-pressure refrigerant to become progressively colder. This in turn results in colder temperatures of the expanded refrigerant entering the subcooler. Depending on the overall system design, refrigerant components in circulation at the cold end of the system, and the operating pressures of the system, it is possible to achieve freezeout temperatures. Since margin must be provided relative to such a condition as freezeout, the resulting refrigeration design will often be limited as the overall system is designed to never encounter a freezeout condition.
Another challenge when using hydrofluorocarbons (HFCs) as refrigerants is that these refrigerants are immiscible in alkylbenzene oil and therefore, a polyolester (POE) (1998 ASHRAE Refrigeration Handbook, chapter 7, page 7.4, American Society of Heating, Refrigeration and Air Conditioning Engineers) compressor oil is used to be compatible with the HFC refrigerants. Selection of the appropriate oil is essential for very low temperature systems because the oil must not only provide good compressor lubrication, it also must not separate and freezeout from the refrigerant at very low temperatures.
U.S. application for patent U.S. Ser. No. 09/894,964 describes a method of freezeout prevention on a very low temperature mixed refrigerant system as referenced in this application. Although this method proved effective for the systems it was employed on, it was not able to provide the required control. This is because, using a valve to increase the pressure of the upstream low-pressure refrigerant to prevent freezeout reduced the refrigeration performance of the system. The disclosed valve has to be adjusted manually, and it is not practical to adjust it manually as needed for the different modes of operation (i.e. cool, defrost, standby and bakeout).
In general a large number of bypass methods are employed in conventional refrigeration systems. These systems, operating typically at temperatures of −40° C. or warmer, employ a single refrigerant component, or a mixture of refrigerants with closely spaced boiling points that behave similar to a single refrigerant components. On such systems, control methods make use of the correspondence between the saturated refrigerant temperature and the saturated refrigerant pressure. On single refrigerant components the nature of this correspondence is such that when a two-phase mixture (liquid and vapor phase) is present, only the temperature or pressure of the refrigerant need be specified to know the other. With mixed refrigerant systems commonly employed, with closely spaced boiling points, small deviations occur from this temperature pressure correspondence but they behave and are treated in a similar fashion as single component refrigerants.
The invention disclosed relates to a very low temperature refrigeration system employing a mixed refrigerant with widely spaced boiling points. A typical blend will have boiling points that differ by 100 to 200° C. For the purposes of this disclosure a very low temperature mixed refrigerant system (VLTMRS) means a very low temperature refrigeration system employing a mixed refrigerant with at least two components whose normal boiling points differ by at least 50° C. For such mixtures, the deviations from single refrigerant components are so significant that the correspondence between saturated temperature and saturated pressure is more complicated.
Due to the added number of degrees of freedom provided by these additional components and the fact that these components behave much differently from each other due to their widely spaced boiling points, the refrigerant mixture composition, the liquid fraction, and the temperature (or pressure) must be specified in order for the pressure (or temperature) to be determined. Therefore, control methods from conventional single refrigerant or mixtures with behavior similar to a single refrigerant, cannot be applied to a VLTMRS in the same manner as conventional systems due to this difference in temperature-pressure correspondence. Although similar from a schematic representation the application of these devices in a VLTMRS is different from prior art due the differences in the pressure temperature correspondence.
As a simple example, conventional refrigeration system controls rely heavily on the fact that controlling the condenser temperature will control the discharge pressure. Therefore, a control valve that controls condenser temperature will control the discharge pressure in a very predictable manner regardless of the mode of operation or the thermal load on the evaporator. In contrast, a VLTMRS using components with widely spaced boiling points will experience large changes in the compressor discharge pressure due to changes in the evaporator load and mode of operation, even if the circulating mixture and condenser temperature are unchanged.
Therefore, some of the schematics shown which embody the invention will be familiar to those practiced in conventional refrigeration. An overview of prior art control methods is given in Chapter 45 of the 2002 edition of the Refrigeration Volume of the ASHRAE handbook. The present system differs from these prior art systems in that the application involves refrigerants with different pressure-temperature characteristics, or more specifically, these refrigerants have no determined pressure temperature correspondence, as do conventional refrigerants. Therefore, the interaction of the control components and the refrigerants is different.
Forrest et al., U.S. Pat. No. 4,763,486, describes a VLTMRS that incorporates an internal condensate bypass. In this method, liquid refrigerant from various phase separators in the process is bypassed to the inlet of the evaporator. The stated purpose of this method is to provide temperature and capacity control of the evaporator cooling, and to provide stable operation of the system. As defined, this method requires flow of refrigerant through the evaporator to provide some level of cooling. No mention is made of a standby mode or a bakeout mode and the schematic clearly shows that the methods shown cannot be used in a standby mode or a bakeout mode. This invention describes the difficulty of starting systems with various numbers of phase separators.
Since the time of this patent, many variations of VLTMRS have been demonstrated, with varying numbers of phase separators, with phase separators that were full or partial separators, and with no phase separators. These demonstrated systems have been successfully operated without utilizing Forrest et al. It is possible that conditions being prevented by Forrest et al. relate to the fact that VLTMRS require a minimal flow rate to support proper two-phase flow of refrigerant. Without adequate flow, the symptoms avoided by Forrest et al. would be expected. Also, Forrest et al. does not make use of a discharge line oil separator. It is known that compressor oil in the VLTMRS can lead to blocking of flow passages and lead to the types of symptoms that Forrest et al. seeks to avoid.
Further, the current application prevents freezeout of the refrigerants in the process. Unlike conventional refrigeration systems where this is not a normal concern, since they typically operated 50° C. or warmer than the freezing points of the refrigerants used in the very low temperature systems disclosed, freeze out is an important consideration.